Characteristics and mechanism of overlying rock and surface damage in gently inclined thick coal seam mining

In order to scientifically guide the disaster prevention and ecological environment protection of the gently coal seam mining in the west of Ordos Basin, and to realize the green and safe mining of coal. The laws of overlying rock and surface damage caused by overlying mining in gently inclined thick coal seam were revealed, and the formation and activity mechanism of surface cracks were expounded by using the methods of similar material simulation, field investigation, crack excavation and theoretical analysis. The results show that the roof overburden of I020213 face extends forward with the repeatable structural evolution characteristics of “cantilever-fracture-hinged joint-collapse rotation-slip-stability”. The failure form of the overlying rock above the goaf was an oblique trapezoid asymmetric migration shape, the compaction degree of the downhill goaf was 1.2 times that of the uphill goaf, and the whole boundary of the goaf was an elliptical parabola similar to parabola. The fracture width of overlying strata has the dynamic characteristics of “increase-decrease”, while the vertical fracture width shows two dynamic characteristics of “increase-stability” and “increase-decrease-stability”. The development height of water-conducting fracture zone with the progressive development characteristic of “rapid increase-slow increase-stability” was 145 m. The surface damage above the goaf was surface uplift, which uplift height was 2.7−36.0 cm. Tensile cracks with average width of 1.5−18.0 cm and bench cracks with average drop height of 17.0 cm were mainly developed above the boundary of the working face. According to the variation characteristics of crack width and depth, the shape of surface fracture profile with skew characteristics was divided into three types: “wedge-like”, “shuttle-like” and “branch-like bifurcation”, also the offset was quadratic polynomial correlation with the fracture development depth. The vertical development depth of the crack less than 3.0 cm is 1.70−3.10 m, and the vertical depth difference was 46.0−190.0 cm/m(the average:132 cm/m), meanwhile the crack depth and width conform to the linear function. The surface uplift above the goaf lags behind the development of mining position and the lag distance is 24.7−71.3 m, the uplift height shows a dynamic process of “increase-stability”, and the active time is 11−13 days. under the action of tension, the boundary cracks show the characteristics of “continuous increase-stability” and “cracking-stability”, and the active time is about 2−12 days. The topsoil layer's characteristics and the distance between cracks have a direct impact on the types of surface cracks that emerge and their activity characteristics. Step cracks without closed activity are often easier to form with greater crack spacing, but tension fractures with closed activity are easier to form.